Dolomitization of Coeval Shelf and Fore-Shelf Carbonates, Lower Permian, Midland Basin, Texas

Abstract

Coeval shelf and fore-shelf carbonates in Lower Permian (Leonardian) rocks in the Midland basin are pervasively dolomitized. Shelf strata consist of a regional mosaic of shelf margin reef, lagoon, and peritidal facies associated with contemporaneous evaporite deposition; fore-shelf facies consist of resedimented shelf margin deposits. Numerous relative sea-level fluctuations that resulted in the formation of Type 1 and 2 unconformities are recognized throughout this section. Mean stable oxygen and carbon isotopic compositions of the shelf strate suggest that dolomitization occurred essentially syndepositionally, likely by interaction with normal marine( ) and hypersaline fluids. In fore-shelf strata petrographic evidence, in combination with paleotemperature estimates from isotopic data and the burial depth-temperature history of these rocks, suggest relatively early (late Permian) dolomitization in rock-dominated systems. Three possible modes of dolomitization could have affected these fore-shelf beds: (1) dolomitization by reflux of normal-marine and/or hypersaline fluids during sea level highstands or lowstands; (2) dolomitization by mixed meteoric-marine fluids during lowstands; and (3) replacement by calcic dolomites via reaction with circulating marine fluids during deposition and early, shallow burial without organic matter influences. The latter mode of origin, however, is favored for most of the rocks examined on the basis of the slightly positive carbonmore » isotopic compositions, Sr versus MgCO{sub 3} compositions (mean Sr 61 ppm, mean MgCO{sub 3} 49.5), and low Mn contents (mean 61 ppm) of the dolomites, although later burial recrystallization is indicated by their relatively depleted isotopic compositions.« less